The present invention relates to a means for accurately detecting and measuring characteristics of signals, and more particularly to rapidly detecting and determining the frequency and other characteristics of signals.
Because of the growing use of the electromagnetic and acoustic spectra in military and civilian application, and in light of the increasing sophistication of modern electronic systems, the need to quickly, economically, and accurately detect and measure characteristics of these signals is becoming increasingly important. It is often necessary to monitor portions of the electromagnetic or acoustic spectra in order to detect sources of energy radiation and determine various characteristics of the radiated energy.
The military especially needs to be able to observe subtle changes in spectral usage that might warn of an impending attack or violations of international laws and agreements. Military ships and aircraft frequently depend for their survival on various types of electronic threat-warning devices. Timely threat-warning is essential in modern warfare in order to locate and attack the enemy, take evasive action, employ countermeasures, or use some combination of these.
Receiving systems currently in use for detection and analysis of signals generally are of one of three types; fixed tuned, manually tuned or automatically tuned. In general, each have for the purpose at hand, inadequate reaction time or poor sensitivity, or both.
Fixed-tuned surveillance receivers generally have very poor frequency resolution because of necessary utilization of wide-band filters in order to cover the frequency range of interest. Conventional tunable surveillance receivers generally "step" or sequence through discrete frequency increments or "channels." These channels are generally of small frequency range with respect to the frequency range of the overall band of the electromagnetic spectrum being examined for signals. The result of the receiver stepping through small frequency increments "listening" for a signal in each increment is that a significant portion of its time is spent tuned to inactive frequencies. Thus there is inherent in a tunable system of this type a large probability of missing signals that are active only briefly, because of the lack of coincidence between the signal frequency and the tuned receiver frequency.
The present invention is based on an application of principles of information theory and results in a system that offers high sensitivity, short response time, and a relatively small number of components. This combination of features allows an important improvement over conventional techniques, and offers a means of overcoming the significant deficiencies inherent in the conventional surveillance receivers described above. Further the invention provides a means for tailoring the performance of the surveillance receiver to the particular application and environment in which it is operating. Importantly then, this invention has the capability of adapting itself, to some extent, to either a static or a dynamic environment.
The present invention is particularly concerned with "monitoring" the greatest number of "channels" in the shortest period of time with a relatively small number of components, while at the same time minimizing signal attenuation and distortion. Clearly there are designs other than that of the present invention which will monitor a greater number of channels in less time, but these designs involve a large number of components or include other undesirable aspects. For example, a design including multiple local-oscillators and a series of corresponding filters of decreasing bandwidth, each of whose output is detected, requires many more components than does the present invention. Further, it introduces undesirable attenuation and distortion, and is not readily usable to obtain information other than the frequency of the monitored signal.
Another design where overlapping of the bands is utilized for resolving the frequency of the monitored signal utilizes equal bandwidth filters, each of a different center frequency. This arrangement also requires more filters than does the present invention. More importantly, this device, too, is not usable to obtain information relating to characteristics of the monitored signal--other than its frequency.